Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique for forming a self-aligned top-gate transistor using an oxide thin film is disclosed (see Patent Document 1).
In addition, a semiconductor device achieving high field-effect mobility (simply referred to as mobility or μFE in some cases) with such a structure that a plurality of oxide semiconductor layers are stacked, the oxide semiconductor layer serving as a channel in the plurality of oxide semiconductor layers contains indium and gallium, and the proportion of indium is higher than the proportion of gallium (see Patent Document 2).